Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF] Department of Physics and Nanotechnology

1. Lecture 4 Dr. Alagiriswamy A A (Sr. Grade) [ M.Sc., PhD, PDF] Department of Physics and Nanotechnology Faculty of Eng. & Technology, SRM UNIVERSITY Main campus, SRM Nagar, Kattankulathur, Chennai – 603203, Tamilnadu Voice : - 91-9791512150 91- 9444128695 Tel : 91-44 -27452270 Extn. 1325

2. Outline of the Presentation

3. Bottom-up approaches

4. Sol-gel processes involve • colloidal particles dispersed in a liquid (sol) • deposition of such particles by spraying, dipping/spinning • they could polymerize (to form a continuous network) • final heat treatments (pyrolyze) to form dense structures • Perhaps this could be “ Bottom – top (up) approach” Tetra ethyl orthosilicate (TEOS)

6. Fabrication procedures; • The SOL is made of solid particles of a diameter of few hundred of nm suspended in a liquid phase. • Liquid solution of organometallic precursors (TMOS, TEOS, Zr(IV)-Propoxide, Ti(IV)-Butoxide, etc. ), which, by means of hydrolysis and condensation reactions, lead to the formation of a new phase (SOL). • Then the particles condense in a new phase (GEL) in which a solid macromolecule is immersed in a liquid phase (solvent). Drying the GEL by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices (XEROGELs). • The fundamental property of the solgel process is that it is possible to generate ceramic material at a temperature close to room temperature. • Therefore such a procedure opened the possibility of incorporating in these glasses soft dopants, such as fluorescent dye molecules and organic chromophores.

7. Fabrication components/details

8. Traditional Sol-Gel Methodology http://www.solgel.com

10. Typical Starting Materials

11. The sol-gel process: (a) sol; (b) gel.

13. Applications of Aerogel Key advantages over other techniques Thermal Insulation Acoustic Insulation Catalyst Support Optical applications Nuclear Waste Storage Filler for paints or others low dielectric constant materials Batteries Ultrafilteration, reverse osmosis • high purity products could be obtained • low temperature processing facilities • Simple, economic methods • Easily shape materials into • complex geometries (gel phase)

14. MICROWAVE SYNTHESIS OF MATERIALS

15. What are microwaves ??? • Microwaves are a form of electromagnetic energy. • Microwaves, like all electromagnetic radiation, have an electrical component as well as a magnetic component. • The microwave portion of the electromagnetic spectrum is characterized by wavelengths between 1 mm and 1 m, and corresponds to frequencies between 100 and 5,000 MHz • absorb the energy, they can reflect the energy, or they can simply pass the energy • Microwave interaction with matter is characterized by a penetration depth and its frequency

16. MICROWAVE SYNTHESIS: - Modern Technology

17. Comparison of conventional heating with microwaves • dipole interactions • ionic conduction Dipole interactions occur loss tangent is the measurable quantity polar ends of a molecule tend to align themselves

18. A close comparison • Conventional heating (thermal) • conduction process • mono-directional • low uniformity • low quality films attainable • Microwaves heating (non-conventional; Advanced) • convection process • multi-directional • high uniformity • so good quality films (Al2O3, Fe2O3, Ti2O3……… attainable

19. Material Synthesis • The discovery of new materials requires the development of a diversity of synthetic techniques. • Microwave methods offer the opportunity to synthesize and modify the composition, structure and morphology of materials, particularly composites viadifferential heating. • Microwave-induced plasmas (MIPs) allow any solid mixture to be heated, and can promote direct microwave heating at elevated temperature, greatly expanding the use of microwaves for reactions between solids and gas–solid mixtures. • Microwave-assisted synthesis is generally much faster, cleaner, and more economical than the conventional methods. • A variety of materials such as carbides, nitrides, complex oxides, silicides, zeolites, apatite, etc. have been synthesized using microwaves.

20. Principle of microwave synthesis; On how • Dipole interactions occur with polar molecules. • The polar ends of a molecule tend to align themselves and oscillate in step with the oscillating electrical field of the microwaves. • Collisions and friction between the moving molecules result in heating. • Broadly, the more polar a molecule, the more effectively it will couple with (and be influenced by) the microwave field.

21. How to quantify this µws procedure ? • Dissipation factor (often called the loss tangent); a ratio of the dielectric loss (loss factor) to the dielectric constant. • The dielectric loss (a measure of how material absorbs)

22. Surface characterization (sophisticated) techniques • SEM/TEM • SFM (AFM/STM and others)

23. Dates • The transmission electron microscope (TEM) was the first type of Electron Microscope to be developed and is patterned exactly on the light transmission microscope except that a focused beam of electrons is used instead of light to "see through" the specimen. It was developed by Max Knoll and Ernst Ruska in Germany in 1931. • The first scanning electron microscope (SEM) debuted in 1938 (Von Ardenne) with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample. Max Knoll (1867 -1968) Von Ardenne (1907 -1996)

24. Electron Microscopy Techniques • Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. • The main advantage of Electron Microscopy is the unusual short wavelength of the electron beams, substituted for light energy ( = h/p) • The wavelengths of about 0.005 nm increases the resolving power of the instrument to fractions • Topography • The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties (hardness, reflectivity...etc.)

25. Features of Electron microscopes Morphology • The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity...etc.) Composition • The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties (melting point, reactivity, hardness...etc.) • Crystallographic Information. How the atoms are arranged in the object; direct relation between these arrangements and materials properties (conductivity, electrical properties, strength...etc.) Monday, September 22, 2014

26. Wings of butterfly Could you name them all Fibrillar hair Red platelet cells

28. A close comparison between Digital microscope Photon beam 2D projection No need of vacuum Difficult to obtain in-situ images Low magnification Inexpensive Electron microscope • Electron beam • 3D projection • need of high/ultrahigh vacuum • Possible in-situ images • high (× 106) magnification • Very expensive ; 50 lacs

30. Types • Scanning electron microscopy, which looks at the surface of a solid object. • Transmission electron microscopy, which essentially looks through a thin slice of a specimen.

31. The "Virtual Source" - the electron gun, produces a stream of monochromatic electrons. • This stream is focused to a small, thin, coherent beam by the use of condenser lenses 1 and 2. The first lens (usually controlled by the "spot size knob") largely determines the "spot size"; the general size range of the final spot that strikes the sample. • The second lens (usually controlled by the "intensity or brightness knob" actually changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam. • The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons (those far from the optic axis, the dotted line down the center) • The beam strikes the specimen and parts of it are transmitted Source : - Inelastically Scattered Electrons Bragg’s law Kakuchi Bands: - Bands of alternating light and dark lines that are formed by inelastic scattering interactions that are related to atomic spacings in the specimen

32. Electrons-solid interactions

33. Scanning Electron Microscope (SEM) Working Concept • SEM allows surfaces of objects to be seen in their natural state without staining. • The specimen is put into the vacuum chamber and covered with a thin coating of gold to increase electrical conductivity and thus forms a less blurred image. • The electron beam then sweeps across the object building an image line by line as in a TV Camera. • As electrons strike the object, they knock loose showers of electrons that are captured by a detector to form the image.

34. Transmission Electron Microscope (TEM) Working Concept • TEM works much like a slide projector. • A projector shines a beam of light through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide. • These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. • This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide. • TEMs work the same way except that they shine a beam of electrons (like the light) through the specimen (like the slide). • Whatever part is transmitted is projected onto a phosphor screen for the user to see. Lanthanum hexaboride (LaB6)

35. SEM Images of Bone tissue

37. Surface characterization (sophisticated) techniques • SFM (AFM/STM and others) “Seeing” at the nanoscale • Vertical resolution 1 Å level • Lateral resolution depends on tip sharpness

38. Branches of Scanning Probe Microscopy http://spm.phy.bris.ac.uk/

39. General Applications • Materials Investigated: Thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors. • Used to study phenomena of: Abrasion, adhesion, cleaning, corrosion, etching, friction, lubricating, plating, and polishing. • AFM can image surface of material in atomic resolution and also measure force at the nano-Newton scale.

40. Scanning Probe Microscopes (SPMs) • Monitor the interactions between a probe and a sample surface • What we “see” is really an image • Two types of microscopy we will look at: • Scanning Tunneling Microscope (STM) • Atomic Force Microscope (AFM) Digital Instruments Multi-Mode head, scanner and base Monday, September 22, 2014 40

41. Putting It All Together • The human hand cannot precisely manipulate at the nanoscale level • Therefore, specialized materials are used to control the movement of the tip SPM system overview

42. Scanning Tunneling Microscopes (STMs) • Monitors the electron tunneling current between a probe and a sample surface • What is electron tunneling? • Classical versus quantum mechanical model • Occurs over very short distances Scanning Probe Tip and surface and electron tunneling

43. Challenges of the STM • Works primarily with conducting materials • Vibrational interference • Contamination • Physical (dust and other pollutants in the air) • Chemical (chemical reactivity)

44. Atomic Force Microscopes (AFMs) • Monitors the forces of attraction and repulsion between a probe and a sample surface • The tip is attached to a cantilever which moves up and down in response to forces of attraction or repulsion with the sample surface • Movement of the cantilever is detected by a laser and photodetector Laser and position detector used to measure cantilever movement Source: http://www.nanoscience.com/education/AFM.html

45. SPM Tips • The size of an AFM tip must be carefully chosen STM tip • Interatomic interaction for STM (top) and AFM (bottom). • Shading shows interaction strength. AFM tip Source: http://mechmat.caltech.edu/~kaushik/park/3-3-0.htm

46. Modes of SFM

47. Different modes of SFM

49. A Nanomechanism A group of grains exchange their neighbors during deformation. Superplasticity – the ability of a material to sustain large plastic deformation – has been demonstrated in a number of metallic, intermetallic and ceramic systems. Conditions considered necessary for superplasticity are a stable fine-grained microstructure and a temperature higher than 0.5 Tm (where Tm is the melting point of the matrix). A compelling competition between crystallization and disordering

50. And What Can We Do? • Using STMs and AFMs in Nanoscience • Allows atom by atom (or clumps of atoms by clumps of atoms) manipulation as shown by the images below Xenon atoms Carbon monoxide molecules Source: http://www.almaden.ibm.com/vis/stm/atomo.html